Logging into Win2000 Srv from NT.40 Wkst

I have installed Win200 Server on our network as a PDC(we was running SBS 4.5)Now all our NT.40 workstations cannot log into the domain.I recieve “A domain contoller for your domain could not be contacted”. I can log in succsully from a 2000 Pro machine. Any Ideas ? Thankyou